You can enable output buffering for all files by setting this directive
to 'On'. If you wish to limit the size of the buffer to a certain size -
you can use a maximum number of bytes instead of 'On', as a value for
this directive (e.g., output_buffering=4096).
As of PHP 4.3.5, this directive is always Off in PHP-CLI.

You can redirect all of the output of your scripts to a function. For
example, if you set output_handler to
mb_output_handler(), character encoding will be
transparently converted to the specified encoding. Setting any output
handler automatically turns on output buffering.

FALSE by default. Changing this to TRUE tells PHP to tell the
output layer to flush itself automatically after every output block.
This is equivalent to calling the PHP function
flush() after each and every call to
print or echo and each and
every HTML block.

When using PHP within an web environment, turning
this option on has serious performance implications and is generally
recommended for debugging purposes only. This value defaults to
TRUE when operating under the CLI SAPI.

User Contributed Notes 1 note

Using "OFF" or no value on output_buffering will disable header modifications, like redirects or content-type or content-disposition resulting in the error we commonly attribute to output before header modifications:

Warning: Cannot modify header information - headers already sent by (output started at C:\PATH\filename.php:1) C:\PATH\filename.php on line 1

Example code with output_buffering = OFF which results in this behavior. Changing it to "ON" or giving it a value will likely cause normal behavior.